By Alina Selyukh WASHINGTON (Reuters) – U.S. presidential candidate Bernie Sanders narrowed the gap with leading Democratic rival Hillary Clinton in a recent opinion poll in the political bellwether state of Iowa as support for the front-runner fell below 50 percent for the first time this year. Clinton, former secretary of state and first lady, remained the top choice for 37 percent of likely Democratic voters in the party’s nomination contest, down from 50 percent in June, according to the Des Moines Register/Bloomberg Politics Iowa Poll released late on Saturday.